The present invention relates to the petroleum refining industry and, more particularly to an alternative approach for alkylating a hydrocarbon feedstock. Specifically, the present invention is an alternative alkylation process involving an acid-dispersion-based alkylation reaction followed by a hydrocarbon/acid-catalyst separation and a continuous on-line regeneration of the total volume of acid catalyst employed in the operation.
Alkylation is a reaction in which an alkyl group is added to an organic molecule. In petroleum chemistry, alkylation involves reacting isobutane or other isoparaffins with olefins in the presence of a strong H. donor, such as a strong acid, in order to produce high octane paraffins which boil in the range of gasoline. In short, an isoparaffin starting material can be reacted with an olefin to produce an isoparaffin product having a higher molecular weight. Industrial alkylation processes generally depend upon the reaction of a C.sub.2 to C.sub.5 olefin with isobutane in the presence of an acid-based catalyst in order to produce an alkylate product. These alkylate products are valuable blending components in the manufacture of gasoline due to their high octane rating and their sensitivity to octane-enhancing additives.
Industrial alkylation processes have historically used hydrofluoric or sulfuric acid catalysts under relatively low temperature conditions. For example, sulfuric acid alkylation reaction is particularly sensitive to temperature, with low temperatures being favored in order to minimize the side reaction of olefin polymerization. Petroleum refinery technology favors alkylation over polymerization because larger quantities of higher octane products can be produced per available light chain olefins. Acid strength in these liquid acid catalyzed alkylation processes is preferably maintained at 88 to 94% by weight using the continuous addition of fresh acid and the continuous withdrawal of spent acid.
Conventional alkylation processes generally take place in three stages. The first stage, known as the reaction stage, takes place in a reactor where hydrocarbon reactants, such as isobutane and olefins, are dispersed into a continuous acid phase. The second stage, known as the separation stage, typically takes place in a settling vessel where the hydrocarbon phase and the acid phase are separated using conventional gravity separation techniques. The third stage involves a heat-exchanging procedure as well as a continuous regeneration of a small portion of the acid-based catalyst using what is known as a "slip stream".
As previously mentioned, conventional alkylation units employed in the oil refining industry typically utilize strong acids, such as HF or H.sub.2 SO.sub.4, as a reaction catalyst. Each of the three stages previously mentioned take place in a separate area or vessel and involve large volumes of acid. For example, in the reactor vessel, hydrocarbon reactants (olefins and isobutane) are dispersed into a continuous acid phase, typically HF, where they react in the presence of the acid catalyst to produce an alkylate product. During the alkylation reaction, a portion of the acid catalyst becomes "spent" or unsuitable for further catalysis of the alkylation reaction. As a result, this spent catalyst must either be regenerated or replaced. In order to minimize costs and maximize efficiency, conventional alkylation techniques utilize large volumes of acid in order to reduce the frequency of acid regeneration and/or acid replacement operations as well as avoiding sudden decreases in acid strength.
Once the alkylation reaction has gone to completion, the reaction mixture is subsequently transferred to a separation vessel where the alkylate product and any unreacted hydrocarbons are separated from the acid phase using conventional gravity separation techniques. Once the separation is completed, the acid phase is removed and transferred to a heat-exchanger where the heat accruing from the exothermic alkylation reaction is dissipated. The acid phase is subsequently returned to the reactor vessel to perform additional catalysis.
Conventional alkylation technology does not utilize the acid only as a proton-donating catalyst for the reaction, but also incorporates the acid to provide a medium for dissipating heat given off by the exothermic alkylation reaction. Larger volumes of the acid catalyst provide more heat-dissipation capability, thereby reducing the necessity for frequent heat-exchanging operations. Consequently, conventional alkylation units also utilize large acid inventories for more efficient heat dissipation.
In addition to its catalytic and heat dissipation roles, the acid catalyst also provides a medium in which undesirable by-products form. The multi-functional role of the acid has further led refinery operators to incorporate large volumes of acid throughout the three stage alkylation process. For example, it is well known that the volume of available functional catalyst decreases as the alkylation reaction proceeds because the acid phase becomes contaminated with undesirable by-products, such as acid soluble oil; i.e. "ASO". Consequently, the acid must be periodically regenerated or replaced otherwise the reaction performance will drop to unacceptable levels.
In a typical regeneration process for HF acid, the acid catalyst is stripped away from the undesirable by-products or contaminants using isobutane and the recovered acid is eventually shunted back to the reaction vessel after undergoing heat-exchange. Generally, the volume of acid catalyst is large enough so that complete regeneration of the catalyst only occurs about twice a day. As previously mentioned, once the hydrocarbon phase, which includes the olefinic and isoparaffinic reactants, has been dispersed into the continuous acid phase and given time to react, the reaction mixture is shunted to a separation or settling vessel where the mixture of acid and hydrocarbons undergoes a gravity induced separation. The acid migrates to the bottom of the vessel and is transferred to a heat-exchanging unit. After adequate heat dissipation has occurred, the acid is subsequently shunted back to the reactor vessel to further catalyze reactions. A small portion of the returning acid catalyst (about 0.01 percent of the total volume) is continuously shunted away to a regeneration zone in what is commonly referred to as a "slip-stream". Once the acid catalyst present in the slip-stream has undergone regeneration, it is returned to the reaction vessel to perform further catalysis.
Sulfuric acid catalysts can not be regenerated by stripping with isobutane. Sulfuric acid catalysts are regenerated by reacting the spent catalyst with oxygen at high temperature to form SO.sub.2 which is then exposed to additional oxygen and converted to SO.sub.3. Fresh sulfuric acid is then made by contacting SO.sub.3 with water.
Conventional alkylation techniques incorporate the methods mentioned above in order to provide the reaction vessel with a constant influx of fresh acid catalyst. In addition to performing a heat dissipation and a catalysis function, this influx of fresh catalyst ensures that the concentration of undesirable by-products does not accumulate to the point that the overall yield of alkylate product is reduced. If the by-product concentration becomes too high, there is a rapid downturn in the yield of alkylate product, because the catalyst will tend to catalyze undesirable reactions between by-products at high concentrations of these contaminants.
As previously mentioned, conventional alkylation technology incorporates large volumes of acid-based catalysts in all three stages of the alkylation operation. This situation is especially problematic when the alkylation procedure incorporates large volumes of noxious liquid acid catalysts, such as HF or H.sub.2 SO.sub.4. An accidental release of an appreciable volume of these acids can have an extremely deleterious effect on the environment.
In particular, large volumes of these acids are employed in the reaction zone in order to dissipate the heat of reaction, to reduce a rapid increase in the concentration of undesirable by-products and to reduce the frequency of acid regeneration and/or acid replacement operations. Additionally, many conventional alkylation processes employ gravity separation vessels which retain large volumes of the hydrocarbon/acid reaction mixture. Anytime a large volume of strong acid is contained at a refinery site for an extended period of time, there is an increased probability for an accidental release of the acid into the environment. Consequently, there is a need for alternative alkylation processes which avoid the environmental and safety concerns associated with conventional alkylation techniques employing large quantities of acid.
While there have been attempts to develop alkylation methods which employ a reduced volume of acid catalysts, these approaches often lack the efficiency necessary for practical implementation into an industrial refinery setting. Typical problems encountered in using a reduced acid inventory include rapid decreases in acid strength, rapid increases in the concentration of undesirable by-products, rapid temperature increases due to the exothermic heat of reaction and rapid increases in the concentration of spent catalyst. Consequently, there is a need for alternative alkylation processes which address the typical problems associated with alkylation techniques employing reduced acid inventories.
It is therefore an object of the present invention to provide an alternative alkylation process utilizing a reduced acid inventory in order to promote safety and minimize the impact of an accidental release of the acid catalyst into the environment.
It is further an object of the present invention to provide an alternative alkylation process which is a viable candidate for implementation into an industrial refinery setting.